The present invention relates to a process for separating the water from an aqueous lactam solution.
It applies more particularly to a mixture resulting from the reaction between an aminonitrile and water (this reaction also being called cyclizing hydrolysis).
During the cyclizing hydrolysis of an aminonitrile in order to form a lactam, one molecule of ammonia per molecule of lactam is also formed. At the end of the reaction, the reaction mixture therefore contains at least the lactam produced, the excess water and ammonia.
It may contain lesser amounts of aminonitrile that has not reacted, or possible by-products of the reaction.
It may also comprise a solvent possibly used in the cyclizing hydrolysis reaction.
The cyclizing hydrolysis may be carried out in vapour phase or in liquid phase. The process of the invention may apply to mixtures resulting from one or other of these methods of preparation.
For the description of the various processes for preparing a lactam by cyclizing hydrolysis of an aminonitrile, reference may be made, for example, to Patent EP-A-0,659,741 or to International Patent Application WO-A-96/22974 or to International Patent Applications WO-A-95/14664 and 95/14665.
The aminonitriles used for preparing lactams are more particularly aminonitriles having from 4 to 12 carbon atoms, preferably linear or branched aliphatic aminonitriles.
As examples of such aminonitriles, mention may be made of those which derive from the hydrogenation of one of the two nitrile functional groups of dinitriles, such as adiponitrile, methylglutaronitrile, ethylsuccinonitrile, dimethylsuccinonitrile, succinonitrile, glutaronitrile and dodecanedinitrile, into a primary amine functional group.
Among the solutions resulting from the cyclizing hydrolysis of an aminonitrile into a lactam, those which correspond to the preparation of caprolactam from 6-aminocapronitrile and water are the most important from an industrial standpoint, since the said caprolactam, upon polymerization, leads to nylon-6.
The process of the invention will therefore relate more particularly to the distillation of the water from aqueous solutions of caprolactam, but this distillation may also be transposed to aqueous solutions of other lactams.
It is necessary beforehand to have separated, generally by distillation, the ammonia formed in the reaction.
One method of separating the water that may be envisaged consists in distilling the water at reduced pressure (less than or equal to 20 mbar absolute, for example) so as not to exceed 145xc2x0 C. at the bottom of the column.
Such a process involves condensing the water thus distilled, by circulating a fluid (such as water) at a temperature of less than or equal to 15xc2x0 C. By way of example, this requires the presence of a refrigeration unit having a power of approximately 1.2 megawatts in order to treat the order of 8 tonnes/hour of an aqueous lactam solution having a solutes concentration of approximately 60% by weight.
The subject of the present invention is a process for separating the water (which process may also be called dehydration) from an aqueous lactam solution, which process is more economical both from the standpoint of the investment necessary for its implementation and from the standpoint of the energy consumed.
More specifically, it consists of a process for separating the water from an aqueous lactam solution, characterized in that:
the water is distilled in a first distillation column by maintaining a temperature at the bottom of the column less than or equal to 160xc2x0 C. and preferably less than or equal to 145xc2x0 C., and at an absolute pressure of 50 millibars to 200 millibars, the distillate being condensed at a temperature of 30xc2x0 C. to 60xc2x0 C. using a coolant system;
the lactam remaining at the bottom of the column is then subjected to a distillation in a second distillation column by maintaining a temperature at the bottom of the column less than or equal to 160xc2x0 C. and preferably less than or equal to 145xc2x0 C., and at a pressure of 10 to 45 millibars, so as to separate the water that it still contains;
the water, containing traces of lactam, distilled in the second column, is recovered in the form of vapour at a temperature greater than or equal to 70xc2x0 C.
By operating the process according to the invention, it is not necessary to have a refrigeration unit for condensing the water at the top of the first column.
The water, containing traces of lactam, distilled in the second column is preferably recycled into the first distillation column. This recycling may be carried out after condensing the water at the pressure of the second column, either between 10 and 45 millibars absolute, corresponding to a temperature of 5xc2x0 C. to 30xc2x0 C. However, such a variant only requires a refrigeration unit having a power of approximately 100 to 200 times less than that of the refrigeration unit which would have been necessary in the context of a distillation using a single column.
According to another embodiment of the invention, the water containing traces of lactam, distilled in the second column may be compressed by means of a steam ejector to a pressure allowing it to condense at a temperature greater than 15xc2x0 C., using a standard coolant such as the atmospheric air or water at room temperature. In this case, the delivery pressure of the steam ejectors must be at least equal to the pressure of the first column, advantageously between 25 millibars absolute and 50 to 200 millibars absolute, the pressure of the first column. In this embodiment, the refrigeration units are omitted.
Overall, the present process is characterized, for the same capacity of distillation of an aqueous lactam solution, by a much lower total energy consumption which can be broken down into an electricity consumption which is 100 to 200 times less, as indicated previously, and an additional energy consumption in the form of steam, in order to heat the second column, which represents only less than approximately 15% of the electrical energy saving made.
The water recovered from the top of the first column is condensed at the pressure of this column, i.e. between 50 millibars and 200 millibars corresponding to a temperature of between 35 and 60xc2x0 C., compatible with most coolants generally used, such as atmospheric air or river water or water from cooling towers.
The water recovered from the top of the second column may be partially condensed at approximately 70xc2x0 C. before being completely condensed and then returned to the first column. This arrangement makes it possible to limit the amount of water to be condensed under particular conditions and therefore to reduce the amount of energy necessary for carrying out this operation.
Before this condensed water is recycled into the first column, it may be advantageous to add a certain amount of water in order to reduce the caprolactam concentration and to prevent it from precipitating.
Furthermore, despite the need for an additional column, the investment necessary, for the same production, is also less in the process according to the invention than in a process carried out using a single column, on account of the considerable refrigerating capacity needed in this latter process.
The aqueous lactam solution, preferably caprolactam, from which the ammonia has been removed, has a lactam concentration which varies widely depending on the conditions under which the said lactam is prepared, especially the initial water/aminonitrile molar ratio and whether or not an organic solvent is present. The lactam concentration may generally vary from 5% to 80% by weight with respect to the total weight of the solution, and preferably from 20% to 75%.
Since the water to be distilled has physical properties that are very different from the lactam from which it must be separated, it is not necessary to use distillation columns having a very large number of theoretical trays.
Columns, particularly packed columns, with at least two theoretical trays are very suitable for implementing the process of the invention. Columns having a number of theoretical trays ranging from 2 to 10 may, for example, be used. The packing used may be of the loose packing or ordered packing type, as proposed by the various manufacturers and sized according to the rules of the art. It is also possible to use columns with trays of the same efficiency, although this is less advantageous from the standpoint of head losses.
The process of the invention does not exclude the possibility of separating the water from the aqueous lactam solution using more than two columns.